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Hmm, that is odd behavior indeed.
I think having keys that point to zero values is important for iterating over a set. For example:
>>> x = Counter(a=10, b=0)
>>> for k in set(x):
... x[k] += 1
... 
>>> x
Counter({'a': 11, 'b': 1})
is probably preferable to
>>> x = Counter(a=10, b=0)
>>> for k in set(x):
... x[k] += 1
... 
>>> x
Counter({'a': 11})
Perhaps to ensure intuitive behavior we could ensure that
>>> Counter(a = 3) + Counter(b = 0) == Counter(a = 3, b = 0)
True
by aggregating all keys into the new Counter object, even those with zero values? I would be happy to make such a patch, as it would be good experience for me to learn. Would this be an acceptable solution, and is there other odd behavior at work here?